Why Do Big Lorries Make A 'Pssssssh' Noise When They Stop?
A lorry stopped and it went "pssssshed" - and my son asked 'what is making the noise?'
James: it was airbrakes.
Amanda: Why is it after they stop?
James: BECAUSE THEY DO (question is left on the board)
Name: Matt, Twickenham
Qualification: HGV Driver
Answer: When a vehicle is driving along it is charging two tanks underneath. As the tanks reach their top pressure - whether stationary or moving along - they are constantly relieving any air not required in the vehicle. Even after they've pulled up or stopped the tanks are still charging - and still releasing excess air - which is the noise you hear. When you're moving you don't hear the noise as there is other noise around.
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